1. Field of the Invention:
The invention relates to light-wave guides and somewhat more particularly to a method of producing light-wave guides comprised of low-light loss fibers having a refractive index which decreases in a radial direction.
2. Prior Art:
A known method of producing light-wave guides (i.e., a light conductive fiber) having a refractive index which continuously decreases toward the peripheral surface of the guide (a so-called gradient fiber) is the double-crucible process. In such a process, a fiber core is drawn from a first crucible filled with a highly transparent molten material (for example, barysilite glass) through a first nozzle and such newly drawn fiber core is then coated with an envelope of another material (for example, a glass having a lower refractive index). Such coating is applied from a second crucible having a second nozzle, generally an annular nozzle receiving the fiber core along its center axis and applying a layer of the other material about the core periphery. Because of the inter-diffusion of the core glass atoms and those of the enveloping glass, a fiber structure is produced having a refractive index which continuously decreases from the refractive index of the core glass toward the refractive index of the envelope glass. While generally, this method is utilized to continuously produce relatively large fiber lengths, there are nevertheless drawbacks in that often contaminants are carried from the crucibles into the ultimate fiber and cause significant light losses and/or scattering. Further, the method is limited to the manufacture of multi-component glass fibers with quickly diffusing glass components.
The CVD (Chemical Vapour Deposition) process is also often used in producing gradient fibers whereby a silica glass hollow tube is coated on its inside with a synthetic glass deposited from a gaseous reaction and the resultant multi-coated tube is then drawn out to form an optical fiber. A mixture of SiCl.sub.4, GeCl.sub.4 and O.sub.2 is typically employed as a gas mixture capable of depositing a SiO.sub.2 -GeO.sub.2 -glass. With this type of process, one attains a non-homogeneous fiber composed of a fiber material which has different chemical compositions on its inside relative to its outside and accordingly exhibits a desired change of refractive index. In general, these types of light-guides have good transmission properties for light signals, however, the process for producing such fibers is disadvantageous because it must be intermittently operated and this can cause periodic as well as irregular refractive index profile fluctuations.